A novel approach to identify accessible and inaccessible pores in gas shales using combined low-pressure sorption and SAXS/SANS analysis

Resolving the pore attributes of gas bearing shale formations is key to precise estimation of the gas-storing capacity of a reservoir. Decades of research and development on the characteristics of shale pores have proven that both organic matter and clay minerals are capable of storing hydrocarbons in their pores by virtue of adsorption; however, very few studies have been able to quantitatively characterize the individual adsorption capacity of clay minerals and organic matter. The volume of the gas that is produced depends on the accessibility to the pores. Due to the very low permeability of shale, there is a significant difference between the accessible pore volume and total pore volume. By general convention, we often term the accessible pore volume as the total pore volume, whereas the total pore volume is usually quite higher than the accessible pore volume. This study attempts to address these two research gaps using Permian age Barakar Formation shales that were collected from two separate sites in the eastern part of India, from a depth range of 221 to 701 m. The shales had a moderately high content of clay (ranging between 25% and 70%) and an abundance of Type III kerogen. The mesopore size distributions were calculated from N-2 adsorption isotherms using a combination of N-2 upon silicate and N-2 upon carbon density functional theory (DFT) model. It shows a 66-100% increase in the cumulative mesopore volume when compared to the conventional N-2 upon carbon model. Micropore size distribution was calculated from CO2 adsorption isotherms using CO2 upon the carbon grand canonical Monte Carlo model, which shows a better fitting than the conventional DFT model. Further, a comparison between small-angle X-ray scattering and low-pressure N-2 adsorption indicates a 3% to 15% higher fractal dimension of total pores than accessible pores. The 'degree of connectivity' of the shale pores was calculated based on the accessible and total pore volumes, which was found to decreasesteadily with depth. Finally, a correlation between the binding potential of the inorganic-organic constituents, and the surface fractal dimension was established using Monte Carlo simulation. We confirmed that the fractal dimension depends on the binding potential of the inorganic-organic matter, and the difference in the fractal dimension of accessible and inaccessible pores could be explained by the smoothening of accessible pores due to pore fluid invasion. This new combined and comprehensive approach will lead to a better understanding of CH4 and CO2 dynamics in shale pores.